Method of electrolytically polishing aluminum and its alloys



atented May 22, 1951" UNITED STATES PATENT OFFICE lllETHOD OFELECTROLYTICALLY POLISH- ING ALUMINUM AND ITS ALLOYS No Drawing.Application January 11, 1949, Serial No. 70,364. In France January 17,1048 4 Claims. (01. 204-1405) It is known to produce surfaces having ahigh reflective capacity on aluminum and aluminum allows by the use ofelectrolytic burnishing or polishing methods. In such methods the partto be burnished is used as an anode in a successive series ofelectrolytic baths; one of those baths is called a, bumishing bath andproduces the burnishing eiiect proper, with or without a polishingaction; a subsequent bath may be called a .re-

nforcing bath and serves to protect the burfiiishcd metal with atransparent film of oxide.

. The steps in the treatment may for example include: 1) Thoroughmechanical bufling;

(2) Cleaning with a solvent or by electrolysis;

(3) Electrolytic burnishing, during which a first film of oxide isformed;

(4) Dissolving said initial oxide film in a phospho-chromic mixture.

(5) Washing in tap-water.

(6) Reenforcement in any one of the usual oxidising baths.

(7) Rinsing in tap-water.

(8) Dyeing.

(9) Filling-in the pit-holes.

(10) Brightening.

My present invention is more especially concerned with the electrolyticburnishing step. While a number of suitable baths are already known someof which are employed on a commercial scale, in practice they all showserious shortcomings. Thus some of the known baths are only satisfactorywhen operated in connection with high-purity aluminum, whereas others onthe other hand are successful only when used on aluminum alloys; inmostcases moreover the operating temperature-of known baths is high, beingclose to 100 C. Another drawback of a number of known baths is that thecurrent density usable therein is too low to permit a rapid burnishingprocess. Then again, they may require a thorough mechanical bufiingtreatment prior to the electrolysis treatment. Another frequent drawbackis that only one face of the part to be burnished may be treated at atime or defects may arise as a result of the treatment, due for instanceto a regular discharge of gases. Finally, known baths may comprise rareand expensive constituent materials, or substances danerous to handle.

I have now discovered that through the use of certain classes ofelectrolytic baths, part or all of the above shortcomings of prior bathsmay be eliminated and a more convenient and simplified electrolytic.burnishing treatment provided for .2 industrial use on aluminum andaluminum containing alloys.

According to the invention, there is provided an electrolyte forelectrolytic burnishing of aluminum and its alloys, which comprises asmain constituents nitric acid and water. The bath may contain onlynitric acid and water. Or alternatively I have obtained satisfactoryresults by adding to the nitric bath an acid or a mixture of acidshaving a high alumina-dissolving capacity. Thus excellent results havebeen obtained when adding horic acid to the nitric bath. Still moresat1sfac u a e a I t adding to the nitric bath a mixture of phosphoricand chromic acids.

I have obtained a handsome polish and burnishing effect when maintainingthe operative conditions within the following ranges:

(1) the nitric acid content within an approximate range from 5% to whena nitric bath is used without any added acid or mixed acids:

(2) when only phosphoric acid is added to the nitric bath, the followingapproximate ranges of compositions are preferably maintained:

(3) When a mixture of phosphoric acid and chromic acid is added to thenitric bath, the approximate preferred ranges are as follows:

Nitric acid from 35% to 45% Phosphoric acid from 8% to 24% Chromic acidfrom 3% to 10% Water as required to make In the above tabulations andthose to follow the percentages are by weight and the acids used areassumed to be anhydrous. In practice we may use:

Nitric acid at 36 Baum, specific gravity d=1.332, containing 52% byweight of nitric acid.

Nitric acid at 40 B., specific gravity d=1.383, Con ining by weight ofnitric acid; this type of acid is used where the bath contains over 52%nitric acid.

Nitric acid at 42 B.. specific gravity d=1.41, containing 92.76% byweight nitric acid; this is used where the bath contains over 62% nitricacid. ,m.

Phosphoric acid-"'at 59 l3., specific gravity d=1.691, containing.Bflfhby weight phosphoric acid.

According to aspects of the invention, the following preferred operativeconditions are em- 3 ployed when using the improved electrolysis bath:

(1) the bath is operated at surrounding temperature. Preferably thetemperature is mamtained under 20 C.

(2) A high degree of agitation is necessary to obtain good results. Thusinsumcient agitation or non-uniform agitation will produce on thealuminum surfaces reflection-distorting fleece-like eflects.

(3) The current density may be varied in the range of from about 90 toabout 450 amperes per square foot with a corresponding time range offrom about to about one or two minutes, according to the current densityand the initial surface condition of the articles.

The cathode used in the improved bath is an aluminum or a graphitecathode. The tanks or containers containing the bath may be made ofstainless steel, various plastics, ebonite-coated iron or generally anysuitable materials impervious to the 'acids used in my process. Coolinmeans should be provided, since heat is rapidly evolved in the bath and,as stated, the temperature thereof should not exceed about C. inoperation.

The steps prior to and the steps subsequent to the bumishing step may beaccomplished in any of the well-known ways. Thus for the solventcleaningoperation, a substance such as trichloroethylene or the like may beused. If the cleaning operation is to be carried out by electrolysis, Imay use, for example, a, bath of the following composition:

Grams Anhydrous trisodium phosphate 100 Anhydrous sodium carbonate 200Soda silica 5 This bath is used at (about 20 C. temperature 'with directcurrent. The articles to be cleaned Phosphoric acid: 360 cc. of an 87%HaPO4 solution (d=1.691)

Chromic acid: 180 grams Water: 640 cc.

The reenforcing treatment may comprise an anodic oxidisation in anaqueous bath containing 20% of its weightof 36 B. sulphuric acid. Thisbath may be used at a temperature of about 20 C. with a current densityof about 13.8

amperes/sq. it.

Some examples will now be given of typical bath compositions accordingto the invention to- Egther with the conditions used in operating Thetime of treatment is from about 10 to 15 minutes, during which strongagitation is maintained.

The bath of this example is particularly suit able for high purityaluminum.

Example 2.Nitric acid: 62%

Water: 38% Conditions of operation:

Temperature: 20 C. Voltage: 4 v. Current density: 280 amp/sq. it.

The time of treatment is from about 10 to 15 minutes, with strongagitation throughout.

The bath of this example is especially suitable for high-purityaluminum.

Example 3.-Nitric acid: 41.5%

Phosphoric acid: 17.5% Water: 41.0% Conditions of operation:

Temperature: 10 to 20 C. Voltage: 3 to 7 v. Current density: 186 to 370amp/sq. ft.

Example 4.Nitric acid: 41%

Phosphoric acid: 14 Chromic acid: 5 Water: 40 Conditions of operation:

Temperature: 10 to 20 C.

Voltage: 3 to '7 v.

Current density: 186 to 370 amp/sq. ft.

(1) The articles to be burnished do not require to be subjected prior toburnishing to a thorough mechanical buffing. since the bath is effectiveto produce in addition to burnishing, a slight polishing or buffingeffect. It is merely necessary to effect a rough mechanical polish inorder to remove the coarser asperities from the surface, such asscratches and surface irregularities found in metal castings direct fromthe foundry.

(2) The improved bath burnishes at room temperature. The fact that it isoperative under 20 C. forms one of the most remarkable characteristicsof the present invention. Thus it will not be required to provide, as inthe conventionally used methods, heating means for the bath container,and this will substantially reduce cost of the plant installation.Moreover the cost of operation 01. the process is also substantiallylowered since it is no longer necessary to hold the bath at hightemperature.

(3) The articles treated are polished and burnished on both faces andtheir appearance is not marred by the presence of stripes due to regulardischarge of gases. This advantage is particularly appreciable inconnection with the manufacture ofarticles used for ornamental purposes.

(4) Use of my improved bath makes it possible to operate under a verylow voltage (about 3 to 7 volts) whereby the item relating to electricenergy consumption in the over-all cost price will be notablydiminished.

The substances used in the composition of the bath are widely available,and may be handled without any exceptional precautions.

(6) The bath is applicable for burnishing both aluminum of high purityand of ordinary grades, as well as aluminum base alloys.

Among the aluminum base alloys in connection with which my improvedelectrolytic burnishing bath is applicable, I may mention aluminumalloys containing copper (such as duralumin) magnesium (such as aluminumalloys containing from 3 to 5% magnesium), magnesium and siliconcontaining alloys such as almasilium, and aluminum alloys containingmagnesium and zinc. A handsome burnish may also be obtained on aluminumalloys of the moldable type, such as those containing from 3 to 5%magnesium, the alloy known as A. P. M. (4.2% Cu, 0.184140% Mg, (LN-0.35%Ti), the alloy known as Y (35-45% Cu, l.2-1.8% Mg, 1.7-2.3% Ni) andothers.

What I claim is:

1. In a method for electrolytically polishing and brightening articlesconsisting at least for the major part of aluminum, the step ofeffecting the electrolytic polishing and brightening treatment at atemperature below 20 C. in an electrolysis bath containing 30 to 45% byweight of nitric acid and a significant amount of at least one member ofthe group consisting of phosphoric and chro'mic acids, the total of theacids ranging between 43 and 79% by weight, the balance being water.

2. In a method for electrolytically polishing and brightening articlesconsisting at least for the major part of aluminum, the step ofeffecting the electrolytic polishing and brightening treatment under acurrent tension of 3 to 7 volts and at a current density of 10 to 50amperes per square decimeter, and at a temperature below 20 C. in anelectrolysis bath containing to 45% by weight of nitric acid and asignificant amount of at least one member of the group consistin ofphosphoric and chromic acids, the total of the acids ranging between 43and 79% by weight, the balance being water.

3. In a method for electrolytically polishing and brightening articlesconsisting at least for the major part of aluminum, the step ofeffecting the electrolytic polishing and brightening treatment at atemperature below 20 C. in an electrolysis bath containing 30 to 45% byweight of nitric acid and 13 to 34% by weight of phosphoric acid, thebalance being water.

4. In a method for electrolytically polishing and brightening articlesconsisting atleast for the major part of aluminum, the step of effectingthe polishing and brightening treatment at a temperature below 20 C. inan electrolysis bath containing 35 to by weight of nitric acid, 8 to 24%by weight of phosphoric acid, and 3 to 10% by weight of chromic acid,the balance being water.

J OS PATRIE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,462,197 Jernstedt Feb. 22, 1949FOREIGN PATENTS Number Country Date 504,026 Great Britain Apr. 17, 1939798,721 France Mar. 11, 1936

1. IN A METHOD FOR ELECTROLYTICALLY POLISHING AND BRIGHTENING ARTICLESCONSISTING AT LEAST FOR THE MAJOR PART OF ALUMINUM, THE STEP OFEFFECTING THE ELECTROLYTIC POLISHING AND BRIGHTENING TREATMENT AT ATEMPERATURE BELOW 20* C. IN AN ELECTROLYSIS BATH CONTAINING 30 TO 45% BYWEIGHT OF NITRIC ACID AND A SIGNIFICANT AMOUNT OF AT LEAST ONE MEMBER OFTHE GROUP CONSISTING OF PHOSPHORIC AND CHROMIC ACIDS, THE TOTAL OF THEACIDS RANGING BETWEEN 43 AND 79% BY WEIGHT, THE BARLANCE BEING WATER.